As the plot shows, the total costs for the production system with partial inert gas area far outweigh the costs of the tunnel type production system.

By Heike Schlessmann, Marketing Manager, and Dr. Ronny Horn, SEHO Systems GmbH, Kreuzwertheim, GermanySoldering in a nitrogen atmosphere can improve solder joint quality and manufacturing yields. Performing circuit assembly in an inert gas atmosphere offers many benefits, especially for lead-free products assembled under high process temperatures, where oxidation and contamination of printed-circuit boards (PCBs) can be avoided in an inert atmosphere. A drawback, of course, is the cost of installing the nitrogen-based environment. Any evaluation of these costs should be weighed against the possible improved quality of the solder joints for a given electronic product and the cost savings that can result due to reduced repair work on manufactured circuits.

Electronic circuits are manufactured under inert gas environments to avoid unwanted reactions with atmospheric oxygen, primarily the oxidation of metal surfaces. Nitrogen is an inert gas that is also the main ingredient of air, at 78.08 percent by volume. Nitrogen can be extracted from air by fractionation — using the different boiling points of the gases that comprise air. Air is liquified by cooling, and its constituent elements are obtained by fractional distillation.

Good environmental gas quality, indicated in terms of volume percent, generally plays a major role in the solder quality of electronic circuits. Nitrogen generators, which can provide gas purities from class 2.5 through class 5.0, are gaining in importance as an alternative to conventional liquid gas supplies. Such generators basically use compressed air guided through a carbon molecular sieve. While the oxygen and other components of air are absorbed, nitrogen molecules are collected in a tank.

These photos compare solder wave results in a good nitrogen atmosphere (13 ppm) at top to a solder wave in a poor nitrogen atmosphere (6000 ppm) at bottom.

In contrast to working with a nitrogen supplier, such nitrogen generators provide a certain degree of independence and possible cost advantages, depending upon the required quality and quantity of the nitrogen being produced. As a rule of thumb, producing nitrogen with a generator is not as cost-effective when the required purity is higher than class 4.5 and if a quantity of nitrogen greater than 103/h is needed.

Avoiding OxidationNitrogen brings about inert reactions due to ternary covalent bonding between the two nitrogen atoms. If components to be joined are maintained in a nitrogen environment, oxygen is not present and any kind of oxidation is avoided. Using a nitrogen environment for electronic manufacturing also has a direct influence on solder spread, wetting force, and angle, as well as on surface tension.

A study performed on solder spread (in C. C. Dong, A. Schwartz, and D. Roth, "Effects of Atmosphere Composition on Soldering Performance of Lead-Free Alternatives," Air Products and Chemicals, Inc., 2010) detailed that solder starts to spread at relatively low temperatures if the rest oxygen level is low. This was found for Sn63/Pb37 materials at a melting temperature of 183°C. In a rest oxygen atmosphere of 10 ppm, solder spreads at 205°C, but only at 270°C in a rest oxygen atmosphere of 1000 ppm. This same effect is not as pronounced for lead-free solder alloys, although a similar trend exists. From this study, it can be assumed that a low rest oxygen atmosphere has a positive effect on solder spread.

The wetting behavior of molten solder describes how it spreads and connects on different surfaces, essentially a determining factor in the quality and form of solder joints in electronic circuits and assemblies. The use of nitrogen is recommended especially for lead-free solder alloys, since Sn/Ag/Cu solder alloys typically show poor wetting characteristics.

An inert atmosphere also has a positive influence on the flow properties of solder due to the reduced surface tension. This can be seen when watching the flowing wave in a wave soldering process. A desired flow pattern can be recognized in a good nitrogen atmosphere even after turning off the wave. The positive effects of a nitrogen atmosphere can benefit both wave soldering processes and solder reflow applications, contributing to larger, more forgiving process windows for both.

Nitrogen in Reflow ProcessesAssemblies soldered using reflow processes in nitrogen have clean appearances because low activation solder pastes can be used and additional cleaning processes are often not needed. When processing fine-pitch components or BGAs, the use of a reflow process in a nitrogen atmosphere can lead to a reduction in defect levels. In a nitrogen atmosphere, the improved wetting and lower surface tension can reduce cold and non-wetted solder joints. The nitrogen atmosphere helps avoid problems due to the "head-in pillow" effect, which is difficult to detect with standard test equipment. This effect is typically caused by excess oxidation, either at the BGA ball or at the surface of the solder paste deposit due to "bleeding" flux.

A miniwave (left) with nitrogen and (right) without a nitrogen atmosphere.

Another study, at the University of Massachusetts in cooperation with several industry partners, explored the influence of a nitrogen atmosphere on soldering defects (in S. Moravec, M. Valenta, and G. K. Arslanian, "Lead-Free Soldering at Inert Atmosphere," European Microelectronics and Packaging Symposium, Prague, Czechoslovakia, 2004). The study found that the total defect rate was reduced by 95 percent in a nitrogen atmosphere.

Still, nitrogen can also enable some issues, such as promoting a tombstone effect, especially when processing two-terminal components. Because of asymmetric paste printing or component placement, two solder paste deposits will melt at different rates. The wetting force of the solder deposit that is molten first is sufficient to draw up the component and, because a nitrogen atmosphere will provide improved wetting force, the effect is intensified. The effect can be counteracted by reducing the temperature gradient while the solder transforms from solid to liquid phase, or by adapting the printing process.

Wave Soldering In NitrogenInert gas atmospheres can be applied to wave soldering processes in different ways. One way is by a partial treatment, in which only the solder wave area is under the inert gas environment. A more effective but more expensive approach is the use of a tunnel system which can operate with an adjustable rest oxygen level between 20 ppm and 500 ppm in the soldering area and also build up an inert atmosphere in the preheat and cooling area. The visible appearance of oxidation is strong in wave soldering processes, and dross must be removed regularly. Typically, the oxidation speed doubles with every 10 K rise in temperature. But tunnel systems as well as wave soldering systems with partial inert atmosphere areas can reduce dross, although the tunnel systems provide drastic improvements compared to partial inert atmosphere systems.

An electronic manufacturer working in a three-shift production operation with both machine types examined and documented the dross appearance over one year. Calculations consider the price for solder alloy as well as refund of costs for dross. While approximately 30 kg of dross had to be removed per week for the system with partial inert atmosphere, the tunnel system yielded only 5 kg oxide dross over the same period. Weekly costs for solder consumption for the system with partial inert atmosphere were computed to be almost six times as much, or 275 Euros compared to only 48 Euros for the tunnel system. The reduction in drossing also implies a reduction in maintenance requirements and an increase in system availability for production. For this example, weekly maintenance time for the system with partial inert atmosphere was about 7.5 h, compared to only 1.1 h for the tunnel system.

When reduction of dross is a main concern in an electronic production process, rest oxygen levels of about 1000 ppm, and a system with a local or partial inert gas atmosphere is fine for soldering. But if all of the advantages of a nitrogen wave soldering process are needed, a rest oxygen level of better than 500 ppm is required, which calls for a tunnel wave soldering system.

The role of flux in a solder process is to remove oxide and polymer layers from metal surfaces and protect them against reoxidation. Most fluxes are washed off when passing the first solder wave. The solder joint, however, is formed directly after passing the second solder wave and the peel-off behavior is decisive for its shape. In an inert environment, aggressive fluxes and additional cleaning processes are not necessary.

Considering all relevant parameters, such as layout, temperature profile, or solderability of the materials being used, the defect rate can be significantly reduced when soldering in a nitrogen atmosphere. This is because the nitrogen atmosphere allows improved wetting characteristics and changing surface tension of the liquid solder. The nitrogen environment helps minimize typical soldering defects such as incomplete through-hole penetration or insufficient wetting but also bridging and solder balls. The low defect rates mean higher product quality and a reduction in repair work.

Production Cost SavingsSome example figures were gathered from an electronics production company. Assemblies processed in a partially inerted wave soldering system required 58.5 h rework per week. A tunnel wave solder machine with continuous rest oxygen level of less than 500 ppm reduced rework time to 34.5 h. Total production costs could show cost savings of more than 120,000 Euros for one year for a tunnel solder system compared to a system with partially inerted environment. Compared to wave soldering machines operating in an ambient environment, the savings would be even more dramatic.

In principle, the same considerations apply for selective wave soldering processes as for conventional wave soldering. A nitrogen atmosphere can positively impact the flow properties of a liquid solder alloy. Controlled solder flow is achieved in a nitrogen environment, but the liquid solder rapidly oxidizes when the inert atmosphere is lost. This loss of control can make it difficult to produce reliable solder connections. In addition, by avoiding oxidation, the lifetimes of the solder nozzles are extended and maintenance is reduced, supporting longer prtoductive periods. Selective soldering systems work at high solder pot temperatures (up to +320°C) which can lead to increased risks of oxidation, but these systems also typically work with minimal nitrogen consumption to control costs.

Whether nitrogen is used in a manufacturing process depends on the application, the materials being used, and the products being manufactured. Nitrogen cannot rescue an improperly managed production process, but it can contribute to a larger process window. The use of nitrogen requires an investment, but using nitrogen may help cut costs and certainly help improve the quality and reliability of solder joints.